In gas-powered furnace systems, sensors of various types are commonly used to provide information for controlling system operation. In residential water heaters, for example, an immersion sensor may be used inside a water tank to monitor water temperature. Commercial water heaters, which typically operate at higher temperatures than residential units, may have a pair of immersion sensors, one at the tank top and one at the tank bottom. Bottom and top sensors typically are monitored relative to a set-point temperature and a temperature range. Heating typically is stopped when the water temperature reaches the set-point temperature and is initiated when the temperature drops below the temperature range.
Water heaters also frequently are configured with flammable vapor (FV) sensors for detecting presence of a flammable vapor. Vapor presence may be detected by using a signal comparator to monitor the resistance level of an FV sensor. For example, where a typical FV sensor resistance might be approximately 10,000 ohms, such resistance could rapidly increase to approximately 50,000 ohms in the presence of a flammable vapor. If the FV sensor exhibits a high resistance as sensed by the signal comparator, gas supply to the heater typically is shut off.
The inventors have observed, however, that FV sensors may undergo changes in resistance due to general ageing, even in a mild environment. Chemical vapors, e.g., chlorines commonly found in household bleaches, can accelerate this process. Over time, a FV sensor may gradually exhibit increased resistance sufficient to cause a false shut-down of a furnace system. On the other hand, the inventors have observed that resistance of a FV sensor may diminish gradually over time, possibly to such a low level that it might not trip a shut-down of a heating system if a flammable vapor event were to occur.
In view of the foregoing, it has become apparent to the inventors that using processor-supplied logic to process sensor inputs and to control heater operation provides opportunities for improving the efficiency and safety of water heater operation. Heating systems are known in which operating power is supplied to a microprocessor by a thermoelectric generator connected to a pilot burner. Such a generator, however, might not be able to generate voltages high enough to operate the processor, unless energy output by the pilot burner is increased.